Simplify the following expression: $9\sqrt{48}$.
Explanation: First, try to factor a perfect square out of the radical. $= 9\sqrt{48}$ $= 9\sqrt{16 \cdot 3}$ Separate the radical and simplify. $= 9\sqrt{16} \cdot \sqrt{3}$ $= 9 \cdot 4 \cdot \sqrt{3}$ $= 36\sqrt{3}$